EP1665867A1

EP1665867A1 - Node for an optical communication network

Info

Publication number: EP1665867A1
Application number: EP04766755A
Authority: EP
Inventors: Olaf Pilcher; Jorg-Peter Elbers; Glen Wellbrock
Original assignee: Marconi Communications GmbH; Ericsson AB
Current assignee: Ericsson AB
Priority date: 2003-09-20
Filing date: 2004-09-09
Publication date: 2006-06-07
Anticipated expiration: 2024-09-09
Also published as: EP1665867B1; CN1883226A; DE10343615A1; ATE536705T1; JP2007506317A; US20070274715A1; WO2005029905A1; JP4903571B2; CN1883226B; JP2012075115A

Abstract

A node for an optical communication network comprises at least one switching unit (2), a plurality of optical interfaces (1) for connecting to a WDM transmission line (3), which comprise a demultiplexer (4) for disassembling a multiplex signal arriving from a WDM transmission line (3) into a plurality of input channels (8), each of which is supplied to an input port of the switching unit (2), and a multiplexer (5) for assembling a plurality of output channels (11), each originating from an out-put port of the switching unit (2), into an outgoing multiplex signal, and at least one transponder (6) for adding an information signal to and dropping it from the communication network, respectively. Input and output branching means (7) between each interface (1) and the switching unit (2) on the path of the input and output channels (8, 11), respectively, are adapted to supply an input channel (8) to the switching unit (2) or to the transponder (6), or to supply an output channel (11) from the interface to the switching unit (2) or the transponder (6).

Description

Node for an optical communication network

The present invention relates to a node for an optical communication network in which several nodes are connected by transmission lines on which optical signals are transmitted in wavelength division multiplex. Such a wavelength division multiplex is formed of a plurality of information signals which are modulated on carrier signals of different wavelengths, and which may originate from different sources and may be bound for different sinks and must therefore be switched independently from one another in a node.

A node for such an optical communication network therefore generally comprises a switching unit, which may be formed of one or more optical switching fabrics and has a plurality of ports for input and output channels, on each of which one of the optical information signals is transmitted. Between the optical switching unit and each transmission line, there is an optical interface which comprises a demultiplexer for decomposing a multiplex signal arriving from the transmission line into a plurality of input channels, each of which is supplied to an input port of the switching unit, and a multiplexer for assembling a plurality of output channels, each of which originates from an output port of the switching unit, into an outgoing multiplex signal. In order to add to the network information signals for transmission and to drop them therefrom, a transponder is provided which brings an information signal supplied by a source into an appropriate form for transmission on the optical network, or, inversely, brings an information signal transmitted on the network into an appropriate form for processing by a sink connected to it. In order to achieve as high a degree of reliability as possible in such a network, all components of the network should be redundant. I.e., for an information signal which is transmitted between start and target nodes of tlie network on a so called working path, at least one protection patli must exist, on which patli a copy of tlie information signal is fransmitted, so that in case of a failure of the main patii the copy fransmitted on the protection path is available at the target node and can be used, or which patli is kept available for fransmitting such a copy in case of need, so that if a failure of the working patli is detected, the transmission may be continued on the protection patli with minimum loss of time.

Working patli and protection path must go along different transmission lines and, if available, different intermediate nodes at tliese transmission lines, so that an interruption of a single transmission line or a failure of a single intermediate node cannot cause working and protection paths to fail at tlie same time.

Start and target nodes are necessarily tlie same for both paths, so that in such a node special measures have to be taken in order to avoid that a partial failure of such a node affects both paths simultaneously.

Known solutions of this problem will be outlined briefly based on Fig. 1, which shows schematically a prior art structure of a node. The node comprises a plurality of interfaces 1, two of which are shown in the Figure, and which connect a central switching unit 2 of the node to bi-directional optical transmission lines 3 attached to the interfaces 1. Each interface 1 comprises a demultiplexer 4 having an input connected to the transmission line and a number of outputs according to the number of wavelengtlis of a multiplex fransmitted on the fransixiission line 3. The demultiplexer 4 disassembles the multiplex signal into its various modulated carrier waves, each of which correspond to one information signal, and outputs these at one of its output ports. Each of these output ports is connected to an input port of tlie switching unit 2, which switches tlie concerned information signal to at least one of its output ports. These output ports are each connected to an input port of a multiplexer 5 which assembles tl e information signals present at its input ports into an outgoing multiplex signal, or to a transponder 6. The transponders 6 each comprise an optical- electrical or an electrical- optical converter which allow a source or sink connected to it, both of which are referred to as a terminal in the following, to input data into the network or to receive them tlierefrom. In order to protect the terminal against failures of tl e transponders 6, there must be assigned to it a working transponder for conveying die working signal and a protection transponder, which conveys tlie protection signal or is adapted to do so in case of need. A protection against failures of the switching unit 2 is not possible sfraightforwardly; if it fails, no optical information signal can be frans- mitted anymore between the transmission lines 3 and the transponders 6.

In order to protect against failures of tl e switching unit 2, it might be duplicated. However, this solution is extremely expensive. A possibility to provide redundancy at less expense is to form the switching unit 2 not as a single switching fabric, tl e input ports of which are connected to all demultiplexers 4 and fransponders 6 and the output ports of which are connected to all multiplexers 5 and fransponders 6, but to form the switching unit from a plurality of switching fabrics, each of which receives from each demultiplexer only a specific carrier wavelength assigned to it and switches it to the multiplexers. If working and protection fransponders of a same terminal are connected to different ones of these wavelength selective switching fabrics, a failure of a single one of these switching fabrics can no longer affect working and protection signals simultaneously. However, tl is solution works only if it is ensured that working and protection signals reach the node not only by different transmission lines, but also with different carrier wavelengtlis.

If d e individual switching fabrics switch not only one but several carrier wavelengths, tl e restrictions are still more serious, since the protection signal, in order to make sure that it is conveyed by anotlier switching fabric than tlie working signal, must not have any of tlie several carrier wavelengths which are switched by tlie switching fabric that conveys tlie working signal.

The object of the invention is to provide a node for an optical communication network which is simple and economic to manufacture and which has no components wliicli are passed through both by a working signal and by its associated protection signal, so that a failure of s component might interrupt both signals. Concerning the aspect of an information signal arriving at the node from a second node for retransmission to a sink connected to the node, the object is achieved by a node having the features of claim 1; concerning tlie aspect of an information signal arriving from a source connected to the node for retransmission to a second node, it is achieved by a node having the features of claim 4. An input branching means wliich is airanged between tlie interfaces and the switching unit allows to supply information signals arriving at the interfaces, in particular a working signal and a protection signal associated to it, which necessarily arrive at different interfaces, to an optical receiver while avoiding the switching unit. Therefore, a failure of the switching unit can only intenτipt those information signals, which go through the node from one interface to another. Since, as indicated above, working and protection signals should not go through tl e same nodes, such a failure can never affect associated protection and working signals. Information signals which are to be dropped at the location of the node do not go through tlie switching unit and are therefore not affected by an eventual failure thereof.

The same applies mutatis mutandis to supplying an information signal from a source into the network at a node. In order to supply working and protection signals of such a terminal, an output branching means is provided between each interface and frie switching unit and is adapted to supply an output channel to the interface either from the switching unit or from the optical transmitter of a transponder. At the node where the information signals are fed into the network, working and protection signals do not go tiirough a same switching unit either. In the simplest case, the branching means might be formed by a signal divider which transmits an information signal coming from a demultiplexer proportionally and simultaneously to an input of die switching unit and to a transponder, respectively, or wliich superimposes output signals of the switching unit and of a transponder. In order to avoid tlie power losses resulting therefrom, tl e branching means are preferably formed as switches which, at a given instant, forward an information signal eitiier only to the switching unit or only to a transponder, or which receive an information signal either only from tl e switching unit or only from the signal converter unit.

Transponders which have one output port of the demultiplexer or one input port of the multiplexer, respectively, assigned to them by which they are adapted to be connected to the input or output branching means, respectively, should be provided in a number coiresponding to that of d e input and output channels, respectively, in order to ensure that an information signal can be dropped or added at any arbitrary wavelength of tlie multiplex.

Transponders which are adapted to supply an information signal with a selectable carrier wavelength to an output channel may be connected to a plurality of output channels by die output brandling means.

In order to allow for bi-dfrectional data traffic, each transponder preferably comprises a transmitter for an output channel and a receiver for an input channel. Transmitters and receivers should then be connected to the branching means of a same interface. An additional advantage is achieved if the branching means not only supply incoming information signals selectively to the branching unit or to one tlie fransponders or receive outgoing information signals selectively either from the switching unit or one of the fransponders, respectively, but are further capable of supplying information signals from a fransponder to the switching unit and vice versa. Thus, e.g., it becomes possible first to extract an information signal, to regenerate it in the fransponder and to supply it again to the network from the same transponder, and, in case die fransponders are tunable, to change the carrier wavelength on which an information signal is transmitted.

Further features and embodiments of the invention become apparent from tl e subsequent description of embodiments, referring to the appended drawings.

Fig. 1, already discussed, shows a conventional node;

Fig. 2 shows a basic configuration of a node according to tlie invention;

Fig. 3 is a first simple embodiment of a branching means that may be used in die node; and

Figs. 4 and 5 show advanced embodiments of branching means.

The node of the invention shown in Fig. 2 as a block diagram is distinguished from the conventional one of Fig. 1 by d e fact tiiat each interface 1 of the node has a branching means 7 associated to it wliich is located at input and output channels 8, 11 between the concerned interface 1 and the input and output ports associated to it of switching unit 2, and which has a number of transponders 6 connected to it.

A first example of the branching means 7 is schematically represented in Fig. 3. N information channels 8, each for one information signal, having carrier wavelengths 11, ..., IN go from an out-put port of demultiplexer 4 of interface 1 to switching unit 2. At each of these input channels 8, a switch, e.g. a moveable mirror 9, is arranged, which, according to its position, eitiier lets the information signal pass to switching unit 2 or deflects it towards receiver 10 of one of tl e transponders 6. A coixesponding group of mirrors 9 is airanged at output channels 11 for carrier wavelengtiis 11, ..., IN wliich go from output ports of tlie switching unit 2 to input ports of the multiplexer 5 of interface 1. In the represented configuration, die input channel 8 which guides d e information signal of carrier wavelengtii 11 is transmitted to a switching unit 2, while the information signal of wavelength IN reaches one of the fransponders 6, where it is converted into an electrical signal and is supplied to a terminal 12. The tirus exfracted information signal is die working signal of terminal 12. The terminal 12 is connected to a further transponder, not shown, which is connected to a branching means 7, which is different from the one represented in Fig. 3 and which receives the protection signal by another interface 1.

The terminal 12 is further connected to a transmitter 13 of die transponder 6 in order to supply a working information signal to tl e network. The transmitter 13 operates at tl e fixed wavelength IN. The optical signal from uiis transmitter 13 reaches a mirror 9, wliich deflects it to an input port of the multiplexer 5 for die wavelength IN. The further transponder, not shown, has a transmitter for transmitting the corresponding protection signal via the other interface. The terminal 12 can tiius transmit a working information signal and a protection information signal without assistance from the switching unit 2.

In this embodiment, for each of the N possible cairier wavelengths, the transponders 6 must be connected to the branching means 7 in order to ensure tiiat any information signal arriving at the branching means 7 can be extracted without regard of its canier wavelengti or that an information signal having die same carrier wavelength can be supplied in d e opposite direction.

The embodiment of Fig. 4 only requires a smaller number of transponders 6. Each of these fransponders 6 has a transmitter 13 which is tuneable to a plurality of carrier wavelengdis li, ..., IN, preferably to all wavelengdis of die multiplex, and in each output channel 11 that corresponds to one of tiiese cairier wavelengths, there is a mirror 9 or a switch having a similar functionality, wliich allows to supply an information signal transmitted by transmitter 13 to one of these output channels 11. Inversely, in each input channel 8 having a coiresponding canier wavelengtii, there is a mirror 9 for extracting an information signal towards receiver 10 of this transponder 6. Since the receivers of conventional fransponders are usually sensitive to all wavelengdis of the multiplex, such a transponder may be connected to any pair of input and output channels 8, 11, according to need.

Fig. 5 is a further advanced embodiment of the branching means 7. This branching means comprises four groups of N by M mirrors, M being the number of transponders 6. A first group of mirrors 9a is for deflecting an incoming infoπnation signal from an input channel 8 to a transponder 6, like in Fig. 4; a second group 9b is for supplying an information signal from a fransponder 6 to an output channel 11. A third group 9c is for deflecting an infoπnation signal from the output channel 11 to one of the fransponders 6, and a fourth group 9d for supplying an infoπnation signal from a fransponder 6 to an input channel 8.

The transponders 6 each have two switches 14, 15 and an impulse shaping circuit 16. In a first position, the switches 14 connect die electrical signal ports of the receiver 10 and d e transmitter 13 of a transponder to ports for a terminal. In a second position they connect tiiem to inputs and outputs of die impulse shaping circuit 16, so that an extracted information signal received by receiver 10 is regenerated electrically in the impulse shaping circuit 16 and is tiien retransmitted as an optical signal by fransmitter 13. Such a fransponder 6 can be used for selectively extracting an attenuated information signal at an input channel 8, to re-amplify it and to supply it again to die same input channel 8 using one of mirrors 9d, in order dien to switch it in the switching unit 2.

Further, it is possible to drop an information signal ti at anives e.g. at an input channel 8 having canier wavelength li, to convert it to another carrier wavelength lj using a transponder 6 and to supply it to d e input channel 8 coπesponding to this wavelengtii. This may be necessary if on the transmission line 3 by wliich the information signal is to be retransmitted from tlie concerned node, the canier wavelength li is already occupied by another signal. As a new carrier wavelength lj, preferably a carrier wavelength which is still available on the outgoing fransmission line 3 is selected, provided that it is also available in d e brandling means 7. If tiiis is not so, the information signal may be wavelength-converted twice, in both branching means 7 it passes on its way through the node.

Claims

1. A node for an optical communication network having at least one switching unit (2), a plurality of optical interfaces (1) for connecting to a fransmission line (3), wliich comprise a demultiplexer (4) for disassembling a multiplex signal airiving from a WDM transmission line into a plurality of input channels (8), each of which is supplied to an input port of d e switching unit (2), and a multiplexer (5) for assembling a plurality of output channels (11), each originating from an output port of die switching unit (2), into an outgoing multiplex signal, and at least one receiver (10) for extracting an information signal from die communication network, characterized in that an input switching means (7) is located between each interface (1) and the switching unit (2) on tl e patii of the input channels (8) and is adapted to supply an input channel (8) to die switching unit (2) as well as to d e receiver (10).

2. The node of claim 1 , characterized in that the input branching means (7) comprises, coπesponding to each output port of the demultiplexer (4), a switch (9) for selectively connecting this output port to one of the input ports of die switching unit (2) or to the receiver (10).

3. The node of claim 2, characterized in that each receiver (10) has one output port of the demultiplexer (4) associated to it, to which it is connectable by d e input branching means (7), and tiiat the receivers (10) are provided in a number coπesponding to the number of d e input channels.

4. A node for an optical communication network having at least one switching unit (2), a plurality of optical interfaces (1) for connecting to a transmission line (3), which comprise a demultiplexer (4) for disassembling a multiplex signal aπiving from a WDM transmission line into a plurality of input channels (8), each of which is supplied to an input port of the switcliing unit (2), and a multiplexer (5) for assembling a plurality of output channels (11), each originating from an output port of d e switching unit (2), into an outgoing multiplex signal, and at least one transmitter (13) for supplying an information signal to the communication network, characterized in tiiat an output switching means (7) is located between each interface (1) and d e switching unit (2) on tl e path of the output channels (11) and is adapted to supply an output channel (11) to d e interface(l) from d e switching unit (2) as well as from the transmitter (13).

5. The node of claim 4, characterized in that the output branching means (7) comprises, coiresponding to each input port of the multiplexer (5), a switch (9) for selectively connecting tiiis input port to one of d e output ports of die switcliing unit (2) or to the transmitter (13).

6. The node of claim 5, characterized in that the transmitters (13) are provided in a number coπesponding to die number of tlie output channels (11), and that each transmitter (13) has one input port of the multiplexer (5) associated to it, to wliich it is connectable by the output brandling means (7).

7. The node of claim 5, characterized in that the transmitter (13) is connectable to a plurality of output channels (11) and is adapted to supply an information signal having a selectable wavelengtii to an output channel (11).

8. The node of claim 1, 2 or 3 and claim 4,5, 6 or 7, characterized by transponders (6), each of wliich comprises one of the transmitters (13) and one of die receivers (10), wherem the transmitter (13) and the receiver (10) of a same transponder (6) is connected to the brandling means (7) of a same interface (1).

9. The node of claim 8, characterized in that the input and output branching means (7) are further adapted to supply to the switching unit (2) an input channel (8) from d e interface (1) as well as to supply it from one of d e fransponders (6), and to supply an output channel (11) from die switching unit (2) to an output channel (11) of d e interface (1 ) and to one of tlie transponders (6).

10. The node of one of d e preceding claims, characterized in tiiat each receiver (10) is an optical-electrical converter and each transmitter (13) is an electrical- optical converter.

11. The node of claim 10, characterized in that at least one fransponder (6) comprises a signal regenerator circuit (16).